The present disclosure relates to a sheet folding device that folds a sheet having an image formed thereon, a sheet post-processing apparatus including the sheet folding device, and an image forming apparatus.
A sheet folding device including a first folding roller, a second folding roller, a conveying path, and a third folding roller is known. The first folding roller and the second folding roller perform a first folding process on the paper (sheet). The conveying path has a stopper that is contacted by the sheet that has been subjected to the first folding process. The second folding roller and the third folding roller allow the sheet, that has been subjected to the first folding process, to enter the conveying path, and perform a second folding process on the sheet. The sheet that has been subjected to the first folding process is tightly folded in its center, and is gently curved and bulged at both ends. Therefore, the conveying path (retreat path) is narrowed at a portion from which the center of the sheet enters. This can stabilize the behavior of the sheet that enters the retreat path.
The sheet folding device described above does not take into account the fact that the overall thickness of folded sheets varies depending on the number of sheets that enter the retreat path. For example, if the retreat path is set to a thickness that allows entry of one sheet that has been subjected to the first folding process, the retreat path cannot therefore accommodate two or more stacked and folded sheets. Also, the sheet folding device described above does not take into account the fact that the thickness of the sheet itself varies from time to time.
Additionally, when the space inside the retreat path is set to accommodate the maximum thickness of sheets that will enter the retreat path, if a thin sheet enters the retreat path, the thin sheet that has been folded may bulge inside the retreat path, or the first folded portion of the sheet that contacts the stopper may move randomly (i.e., behave unstably) inside the retreat path. If this sheet is then subjected to the second folding process, the position of the second fold of the sheet may deviate or the second fold may be skewed.